Fix PretrainedTokenizerFast check => Fix PretrainedTokenizerFast Save (#35835)

* Fix the bug in tokenizer.save_pretrained when saving tokenizer_class to tokenizer_config.json

* Update tokenization_utils_base.py

* Update tokenization_utils_base.py

* Update tokenization_utils_base.py

* add tokenizer class type test

* code review

* code opt

* fix bug

* Update test_tokenization_fast.py

* ruff check

* make style

* code opt

* Update test_tokenization_fast.py

---------

Co-authored-by: Qubitium-ModelCloud <qubitium@modelcloud.ai>
Co-authored-by: LRL-ModelCloud <165116337+LRL-ModelCloud@users.noreply.github.com>

src/transformers/tokenization_utils_base.py

@@ -2472,8 +2472,8 @@ class PreTrainedTokenizerBase(SpecialTokensMixin, PushToHubMixin):
 
         # Add tokenizer class to the tokenizer config to be able to reload it with from_pretrained
         tokenizer_class = self.__class__.__name__
-        # Remove the Fast at the end unless we have a special `PreTrainedTokenizerFast`
+        # Remove the Fast at the end if we can save the slow tokenizer
-        if tokenizer_class.endswith("Fast") and tokenizer_class != "PreTrainedTokenizerFast":
+        if tokenizer_class.endswith("Fast") and getattr(self, "can_save_slow_tokenizer", False):
             tokenizer_class = tokenizer_class[:-4]
         tokenizer_config["tokenizer_class"] = tokenizer_class
         if getattr(self, "_auto_map", None) is not None:

tests/tokenization/test_tokenization_fast.py

@@ -20,7 +20,7 @@ import shutil
 import tempfile
 import unittest
 
-from transformers import AutoTokenizer, PreTrainedTokenizerFast
+from transformers import AutoTokenizer, LlamaTokenizerFast, PreTrainedTokenizerFast
 from transformers.testing_utils import require_tokenizers
 
 from ..test_tokenization_common import TokenizerTesterMixin
@@ -170,6 +170,41 @@ class PreTrainedTokenizationFastTest(TokenizerTesterMixin, unittest.TestCase):
             # thus tok(sentences, truncation = True) does nothing and does not warn either
             self.assertEqual(tok(sentences, truncation = True, max_length = 8), {'input_ids': [[8774, 6, 3, 63, 31, 1748, 55, 1],[ 571, 33, 25, 3, 2, 3, 58, 1]], 'token_type_ids': [[0, 0, 0, 0, 0, 0, 0, 0],[0, 0, 0, 0, 0, 0, 0, 0]], 'attention_mask': [[1, 1, 1, 1, 1, 1, 1, 1],[1, 1, 1, 1, 1, 1, 1, 1]]})  # fmt: skip
 
+    def test_class_after_save_and_reload(self):
+        # Model contains a `LlamaTokenizerFast` tokenizer with no slow fallback
+        model_id = "deepseek-ai/DeepSeek-R1-Distill-Qwen-1.5B"
+
+        with tempfile.TemporaryDirectory() as temp_dir:
+            tokenizer = AutoTokenizer.from_pretrained(model_id, use_fast=True)
+            self.assertTrue(
+                isinstance(tokenizer, LlamaTokenizerFast),
+                f"Expected tokenizer(use_fast=True) type: `LlamaTokenizerFast`, actual=`{type(tokenizer)}`",
+            )
+
+            # Fast tokenizer will ignore `use_fast=False`
+            tokenizer = AutoTokenizer.from_pretrained(model_id, use_fast=False)
+            self.assertTrue(
+                isinstance(tokenizer, LlamaTokenizerFast),
+                f"Expected tokenizer type(use_fast=False): `LlamaTokenizerFast`, actual=`{type(tokenizer)}`",
+            )
+
+            # Save tokenizer
+            tokenizer.save_pretrained(temp_dir)
+
+            tokenizer = AutoTokenizer.from_pretrained(temp_dir, use_fast=False)
+            # Verify post save and reload the fast tokenizer class did not change
+            self.assertTrue(
+                isinstance(tokenizer, LlamaTokenizerFast),
+                f"Expected tokenizer type: `LlamaTokenizerFast`, actual=`{type(tokenizer)}`",
+            )
+
+            tokenizer = AutoTokenizer.from_pretrained(temp_dir, use_fast=True)
+            # Verify post save and reload the fast tokenizer class did not change
+            self.assertTrue(
+                isinstance(tokenizer, LlamaTokenizerFast),
+                f"Expected tokenizer type: `LlamaTokenizerFast`, actual=`{type(tokenizer)}`",
+            )
+
 
 @require_tokenizers
 class TokenizerVersioningTest(unittest.TestCase):